1. Field of the Invention
Embodiments of the invention relate to an organic light emitting display and more particularly to an organic light emitting display capable of sensing degradation of an organic light emitting diode.
2. Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (OLEDs) capable of emitting light by themselves, and bears advantages such as a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, and the like.
The OLED serving as a self-emitting element generally includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer may include a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL may move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
The organic light emitting display may arrange pixels, each including an OLED, in a matrix form and adjust a luminance of the pixels depending on grayscale of video data. Each pixel may include a driving thin film transistor (TFT) which controls a driving current flowing in the OLED depending on a gate-to-source voltage Vgs between a gate electrode and a source electrode of the driving TFT. A display grayscale (e.g., a display luminance) may be adjusted by an emission amount of the OLED proportional to a magnitude of the driving current.
The OLED may generally have a degradation characteristic of an increase in an operating point voltage (e.g., a threshold voltage) of the OLED and a reduction in an emission efficiency as an emission time of the OLED passes. Because an accumulated value of currents applied to the OLED of each pixel may be proportional to an accumulated value of gray levels represented in each pixel, the OLEDs of the pixels may have different degradation degrees. A degradation deviation between the OLEDs of the pixels results in a luminance deviation, and an image sticking phenomenon may be generated by an increase in the luminance deviation.
A related art compensation method for sensing the degradation of the OLED and then modulating video data based on a sensing value using an external circuit is proposed to compensate for the degradation of the OLED. In the related art compensation method, a data driving circuit directly receives a sensing voltage from each pixel through a sensing line and converts the sensing voltage into a digital sensing value. The data driving circuit then transmits the digital sensing value to a timing controller. Further, the timing controller modulates digital video data based on the digital sensing value and compensates for the degradation deviation of the OLED.
The related art compensation method has problems. The related art compensation method adopts a voltage sensing method to sense the degradation degree of the OLED. For example, the related art compensation method stores an anode voltage of the OLED in a parasitic capacitor of the sensing line and then senses the stored anode voltage of the OLED. In this instance, because a parasitic capacitance of the sensing line is very large, for example, several hundreds to several thousands of picofarads (pF), the time required in a sensing operation necessarily increases. For example, when the parasitic capacitance of the sensing line is large, it takes much time to charge the parasitic capacitor at a voltage level capable of being sensed. The problem is more serious in the sensing operation of a low gray level than a high gray level.
Further, the parasitic capacitance of the sensing line may vary depending on design conditions of the display panel affected by data lines adjacent to the sensing lines. When the sensing lines have different parasitic capacitances as described above, it may be difficult to obtain an accurate sensing value.